1. Field of the Invention
The present invention relates to a control unit and a method of manufacturing the same for an electric power steering device which is more suitable to be mounted particularly on the relatively large vehicles, or to a control unit for a similar device.
2. Description of the Related Art
Generally, an electric power steering device has a torque sensor for sensing a steering torque produced on a steering shaft by the operation of a handle, and an assist motor (hereinafter simply referred to as a motor) attached to the steering shaft to produce a steering auxiliary torque bypassing an electric current from a battery of a vehicle to the assist motor in accordance with the sensed steering torque. This electric power steering device is difficult to produce a large steering auxiliary torque, in contrast to a hydraulic type, and conventionally was employed mainly in the light cars. However, this electric power steering device has some advantages that electronic control is easy, or the hydraulic pump or hydraulic pipe is unnecessary, and the structure is simple. In recent years, it is examined that the electric power steering device is applied to the small cars having a displacement level of 1800 cc, and will be possibly applied to the large cars in the future.
For the current control of the assist motor in this electric power steering device, a drive circuit having an H bridge circuit composed of normally four (or four pairs) FETs (Field Effect Transistors) is used to drive the assist motor in accordance with a PWM (Pulse Width Modulation) method under the control of a control circuit comprising a microcomputer. Also, the electric power steering device of this kind has a power back-up capacitor normally consisting of an electrolytic capacitor, which is connected to a high potential power line for supplying an electric power from the battery of the vehicle to the drive circuit. The power back-up capacitor resolves the problems with a ripple current or a temporary power voltage drop that may be caused by wiring resistance when a large current flows. Also, the drive circuit, a control circuit, and a large current circuit component for the power back-up capacitor are mounted on a circuit substrate within one unit, referred to as a control unit. This control unit (hereinafter simply referred to as a unit) was hitherto disposed in a space which was hidden from the driver within a compartment of the vehicle.
The prior art control unit for the electric power steering device had a structure of accommodating one sheet of circuit substrate within a metallic case, in which the circuit substrate was constructed as shown in FIG. 6, for example. That is, its basic structure is the same as that of a so-called printed board, in which the circuit conductors or passive elements are formed as a wiring pattern by the printed wiring technique on an insulating substrate made of resin. The IC chips or parts such as the FET or power back-up capacitor to constitute the control circuit are mounted thereon.
In this connection, a circuit substrate 1 as shown in FIG. 6 comprises a FET chip 2 molded by resin, a power back-up capacitor 3, and an IC chip of a microcomputer 4 constituting the control circuit. The circuit substrate 1 further comprises a bus bar 5 (conductor plate), a heat radiator 6, and a relay 7. The bus bar S constitutes a circuit conductor such as a conduction line of the assist motor through which a large current flows. The heat radiator 6 radiates the heat generated mainly in the FET 2. The relay 7 opens or closes the conduction line of the assist motor under the control of the microcomputer 4.
Since a current of about 25 A at maximum flows through the assist motor in the light cars, it was practically impossible, due to the reasons of the heat generation or the space on the substrate, to constitute the conduction line of the assist motor using a wiring pattern on the circuit substrate 1 that is limited in the thickness. Therefore, the conduction line was constituted by attaching the bus bar 5, which was considerably larger in width and thickness than the wiring pattern on the circuit substrate 1, on the circuit substrate 1. The quantity of heat generated in the FET 2 is very large depending on the working conditions, and if left alone, the substrate rises in temperature to about several hundreds xc2x0 C. in short time. Hence, the radiator 6 was fabricated of aluminum having high heat conductivity, and placed in parallel with each FET 2 at the end of the circuit substrate 1, while being bonded with a back face of the FET 2.
By the way, the prior art control unit for the electric power steering device had the following problems with the applicability or on-board capability particularly for the relatively large vehicles.
(i) In prior art, the parts were arranged in plane on one sheet of circuit substrate, so that the size of the circuit substrate became large in a planar direction and the weight was heavy. In particular, in the case of the relatively large vehicles such as the small cars, the current of the assist motor reaches a maximum of 60 to 80 A, making the radiator or bus bar extremely large and heavy. This is directly reflected on the whole size or weight of the unit, resulting in extremely large and heavy unit as a whole. And in the case of the small cars, there is an unnegligible problem with the voltage drop which is caused by the wiring resistance between the unit and the motor owing to increased current of the assist motor. In the situation where the wiring can not be thickened owing to the costs, it is required in practice that the control unit is placed near the assist motor (i.e., within an engine room) However, it is significantly difficult to place it in a space within the engine room, if the unit is large, and further to apply the device itself to the small cars due to the high costs or the limited space.
(ii) To apply the device to the small cars, in the case where the unit is placed within the engine room owing to the necessity as described above, the unit must have a waterproof structure to withstand the submersion under water. However, in prior art, such water resistance was not considered at all, resulting in a problem that such waterproof structure must be constructed inexpensively. Since the water does not normally permeate into the car room, too high water resistance is not required if the unit is disposed within the car room. However, in the case where the unit is placed within the engine room (i.e., beneath the engine room in which the assist motor is placed), the water may possibly permeate into the engine room, whereby the unit must have a water resistance to withstand the submersion under water.
(iii) In prior art, the circuit components such as a power back-up capacitor or a relay through which a large current flows were mounted on one sheet of circuit substrate consisting of a normal printed board typically by soldering the terminals. Particularly in the case where the device is applied to the small cars to increase the current, the solder melts on the connecting portion of this terminal, resulting in a problem that the possibility of improper connection can not be ignored.
In Japanese Patent Unexamined Publication No. Hei. 6-270824 or Japanese Patent Unexamined Publication No. Hei. 8-11732, to solve the above-described problem (i) (larger size of the unit), an electric power steering circuit device (i.e., a control unit for the electric power steering device) was disclosed in which a drive circuit having a bridge circuit comprising FET is mounted on a metal substrate (or a radiating plate), and a control circuit containing a microcomputer is mounted on a separate insulating substrate (i.e., a normal printed board), the metal substrate and the insulating substrate being disposed one on another with a predetermined spacing. With this device, two substrates each having classified circuit components are laminated, giving rise to the effect of smaller size.
However, this device has a structure of simply laminating two substrates, attaching circuit components such as a power back-up capacitor to a support member interposed between the substrates, and soldering the terminals of circuit components such as the power back-up capacitor to the wiring pattern on the support substrate. Therefore, the space of disposing at least the circuit conductor corresponding to the prior art bus bar 5 as described previously has not be reduced, and further is expected to be reduced. Also, the problem (ii) with the water resistance and the problem (iii) with the reliability of connection have not been solved at all.
The problem (ii) with the water resistance is not limited to the control unit of the electric power steering device, but also may arise with the control unit for the other devices (e.g., an engine control system, an AT control system, an anti skid braking system (ABS system) or a power window system) mounted on the vehicle. In recent years, particularly there is the increasing demand for safer vehicles, whereby each device is required to have the higher reliability when the vehicle is submerged under water. Even with the control unit disposed within the vehicle compartment, there is the trend of demanding for the high waterproof performance.
The present invention has been achieved in the light of the above-mentioned problems, and it is an object of the invention to provide a control unit and a method of manufacturing the control unit for an electric power steering device with the enhanced applicability or on-board capability particularly for the relatively large vehicles.
Moreover, it is another object of the invention to provide a control unit which can maintain the high water resistance when submerged under water.
According to the present invention, there is provided a control unit for an electric power steering device that uses an assist motor linked to a steering system of a vehicle to generate a steering auxiliary torque, comprising a drive circuit containing a switching element for switching the connection of each coil terminal of the assist motor to a high potential power line or a low potential power line, a control circuit for controlling the operation of the assist motor by controlling the operation of the switching element of the drive circuit, a large current circuit component containing a power back-up capacitor connected to the high potential power line, a metal substrate made of a metal and having the drive circuit mounted, an insulating substrate made of an insulating material and having the control circuit mounted, and a baseboard having a metallic circuit conductor constituent member integrated with a resin substance by insert molding and the large current circuit component mounted, wherein the metal substrate and the insulating substrate are attached on the baseboard to overlap each other.
Herein, a xe2x80x9cdrive circuitxe2x80x9d mounted on the metal substrate may contain a bridge circuit for PWM driving the assist motor. Also, a xe2x80x9cswitching elementxe2x80x9d may be an FET constituting the bridge circuit for PWM driving the assist motor. A xe2x80x9ccontrol circuitxe2x80x9d mounted on the insulating substrate comprises, for example, a microcomputer or its peripheral circuits (through which no large current flows). A xe2x80x9clarge current circuit componentxe2x80x9d mounted on the baseboard may be a ceramic condenser for countermeasure of the electric wave to suppress the noise emission, or a relay that is controlled by the control circuit to open or close the conduction line, besides the power back-up capacitor (normally an electrolyte capacitor). It is needless to say that the circuit components through which no large current flows may be mounted on the baseboard. Also, a xe2x80x9cmetallic circuit conductor constituent memberxe2x80x9d may be a metallic plate member subjected to press working.
With this invention, the circuits or parts are mounted on the corresponding substrates that are optimal for each function, and the substrates are laminated on the baseboard. Therefore, the unit can be reduced in size, and mounted on the vehicle more suitably.
That is, first of all, the drive circuit containing a switching element for switching the energizing state of the assist motor and having a large quantity of heat generated is mounted on the metal substrate having excellent heat conductivity and secured for high ability of heat radiation. Thereby, the width or interval of a wiring pattern on the metal substrate making up the circuit conductor of this drive circuit can be set to be narrower than the prior art, resulting in smaller area of the drive circuit mounting portion or the whole metal substrate.
The control circuit having less current flowing is mounted on the ordinary insulating substrate, and can be disposed within a minimum of area.
And the large current circuit component containing a power back-up capacitor is mounted on the baseboard having a circuit conductor made up by insert molding a metallic member. Namely, the circuit conductor through which a large current flows, which was constructed by the bus bar on the insulating substrate in prior art, is integrated with a resin base substance of the baseboard by insert molding the metallic member. Therefore, by increasing the set thickness of this metallic member, the circuit conductor for passing the large current can have a sufficient capacity (cross sectional area) while the size of the circuit conductor in a planar direction is suppressed to a small value. Further, since this circuit conductor is buried into the baseboard by insert molding, the space for arranging a mounting portion of the large current circuit component in a thickness direction can be reduced, as compared with when the bus bar is provided on the substrate.
Moreover, since the metal substrate and the insulating substrate are laminated on the baseboard, the size of the whole unit in a planar direction can be reduced significantly.
Accordingly, the unit can be reduced in size in a planar direction, as well as in a thickness direction to equivalent to or less than the size in prior art, and correspondingly the weight can be decreased. Hence, for the relatively large vehicles having a larger current amount of the assist motor, the unit can exhibit the enhanced applicability or on-board capability in the respects of the space and weight.
According to a preferred embodiment of this invention, a heat radiating plate is disposed to constitute an outside wall of the unit, a back face of the metal substrate being bonded with an inside face of this heat radiating plate.
With this embodiment, the higher ability of radiating the heat generated in the drive circuit can be secured, while the increasing number of parts is suppressed. Further, there is the advantage that mounting the metal substrate on the baseboard or its electrical connection can be facilitated. Because the heat radiating plate constitutes the outside wall, there is no need of providing a cover member making up the outside wall in an area where the heat radiating plate is provided. As a result, an outside face of the heat radiating plate is exposed to the outer atmosphere, giving rise to the higher ability of radiating the heat. For example, the metal substrate itself can be also constructed to also act as a heat radiating member. In this case, the metal substrate itself is required to have a heat radiating area in the respect of size or shape, it being apprehended that mounting the metal substrate on the baseboard and its electrical connection can not be necessarily facilitated. However, if the heat radiating plate and the metal substrate are separated, such a problem will not arise.
According to a further preferred embodiment of this invention, an electrode of the large current circuit component is welded to a predetermined region of the circuit conductor constituent member of the baseboard.
With this embodiment, Even when its connecting portion is heated owing to a large current flowing through the large current circuit component, there possibly occurs no inconvenience such as solder melting, whereby the large current circuit component provides a high reliability of connection. Accordingly, the unit can be further enhanced in applicability to the large vehicles in the aspect of the higher connection reliability of the large current circuit component.
According to a further preferred embodiment of this invention, a frame-shaped resin case is placed to surround the baseboard and constitute an outside wall of the unit, the baseboard being substantially a sheet, and accommodated within the resin case.
With this embodiment, the mounting operation of the large current circuit component on the baseboard can be facilitated, and the baseboard or the outside wall surrounding it can be manufactured more easily. For example, a wall face portion to make the outside wall can be provided integrally with an outer peripheral portion of the baseboard, but in this case, the baseboard is shaped unlike a simple sheet, and surrounded by the wall face portion, the wall face portion hindering the work, thereby making it difficult to mechanically implement the mounting or connecting operation of the large current circuit component, and resulting in the more costs. Also, the fabrication of the baseboard itself is relatively more difficult. However, this embodiment can eliminate such problems. Accordingly, the manufacturing costs can be reduced, and the applicability of the unit to the large vehicles can be enhanced.
According to a further preferred embodiment of this invention, the resin case is provided with a connector for the outside wiring, in which a predetermined electrode of this connector and a predetermined region of the circuit conductor constituent member of the baseboard are connected by screwing the terminals that mate with each other.
With this embodiment, there is no inconvenience of making a false connection owing to the welding flag or solder ball produced, when the welding or soldering is performed. As a result, there is the advantage that the unit can be enhanced in the operation reliability, and the connecting operation between the circuit conductor of the large current circuit component and the connector can be facilitated as compared with the welding or soldering.
According to a further preferred embodiment of this invention, the resin case is provided with a connector for the outside wiring, and the terminals for connecting a predetermined electrode of this connector and a predetermined region of the circuit conductor constituent member of the baseboard to a predetermined circuit conductor of the insulating substrate are disposed on one side of the insulating substrate.
Herein, a xe2x80x9cterminalxe2x80x9d means a portion for making the electrical connection of conductors or electrodes that are joined or engaged (fitted) with each other, or in some cases, by soldering, screwing, or welding. For example, the splicing pieces that are joined together, or a concave portion (including a through hole) and a convex portion (e.g., a projection or a pin) that are fitted with each other.
With this embodiment, the circuit connection of the baseboard with the insulating substrate having the control circuit mounted (i.e., connection of the circuit through which no large current flows, mainly, the signal line) is made simply on one side of the unit. Therefore, the operation of incorporating the insulating substrate having the control circuit mounted, or the operation of welding or soldering the terminals required in some cases after that incorporating operation can be easily mechanized.
The connection of this insulating substrate can be effected using a so-called press fit terminal, because the connecting circuit has no large current flowing. If the connecting terminals are collected on one side of the insulating substrate, the connecting operation is facilitated even in the case where the press fit terminal is employed. Note that the xe2x80x9cpress fit terminalxe2x80x9d comprises one terminal like a pin and the other terminal being a through hole (i.e., a terminal like a through hole), the conduction state being secured by press fitting the pin-shaped terminal into the through hole terminal. For this press fit terminal, there is no need of welding or soldering, resulting in easier connecting operation.
According to a further preferred embodiment of this invention, the resin case is provided with a connector for the outside wiring, the connection of a predetermined electrode of the connector and a predetermined region of the circuit conductor constituent member of the baseboard with a predetermined circuit conductor of the insulating substrate is effected by one terminal provided on a face of the resin case and the baseboard opposed to the insulating substrate, and the other terminal that can engage or join with the one terminal upon an operation of attaching the insulating substrate on the baseboard, the other terminal being provided on a face of the insulating substrate opposed to the resin case and the baseboard.
With this embodiment, an intermediate assembly containing the baseboard and the resin case (sub-assembly) is first assembled, and then the insulating substrate is attached to the baseboard while the terminals on both sides are engaged or joined, and then the terminals are soldered together, as required. In this way, the assembling of the baseboard, the resin case and the insulating substrate is completed, and at the same time the electrical connection of the insulating substrate with the baseboard and the resin case (connector) is completed, resulting in improved assembling operation of the unit. In this case, if the metal substrate is pre-assembled with the baseboard and the electrical connection is completed, the assembling of the metal substrate, the baseboard and the insulating substrate, and the electrical connection between them are all completed at once.
According to a further preferred embodiment of this invention, the resin case is provided with a connector for the outside wiring, the connector comprising a fitting portion formed integrally with the resin case and into which another connector is fitted, and an electrode provided integrally in the resin case by insert molding.
With this embodiment, the fitting portion is securely prevented from being unsteady or shifted, whereby the operation of inserting or extracting the male connector (on the external equipment side) into or out of the female connector (on the unit side) can be effected stably. Also, the number of parts can be decreased in contrast to the case where the connector is provided as a separate part.
According to a further embodiment of this invention, an opening is formed in the baseboard where the metal substrate is attached to expose at least a mounting surface of the metal substrate to an opposite side of the baseboard where the metal substrate is attached, an inner surface of the opening surrounding the mounting surface of the metal substrate to form a concave portion for filling the molding material. Herein, the xe2x80x9cmounting surfacexe2x80x9d as above referred to means an area on the surface of the metal substrate where at least the drive circuit is mounted.
With this embodiment, by pouring and fixing a predefined amount of molding material into the concave portion (e.g., thermosetting resin), the mounting surface of the metal substrate in which the circuit including at least the drive circuit is formed can be molded excellently.
According to a further preferred embodiment of this invention, a predetermined region of the circuit conductor constituent member of the baseboard and a predetermined circuit conductor of the metal substrate are connected by wire bonding.
With this embodiment, the problem does not arise with the soldering ball or welding flag, which may occur on the structure in which the terminal is stood on the metal substrate and connected to the baseboard by soldering or welding, thereby resulting in the higher reliability. The connecting operation of the metal substrate leads to the higher productivity or mass production capability.
According to a further preferred embodiment of this invention, an insulating cover member is provided as an outside wall of the unit which covers at least an opposite face of said baseboard where said metal substrate is attached. Herein, the insulating cover member may be made of synthetic resin, for example.
With this embodiment, there is no need of securing a predefined insulation distance between the circuit component or circuit conductor inside the unit and the outside wall of the unit, so that the outside wall of the unit can be disposed more inside, contributing to the smaller and enhanced on-board unit for the vehicle. Since the prior art unit of this kind had the outside wall consisting of a cover member normally made of metal, it was required to provide a sufficient clearance beyond a predefined insulation distance with respect to the circuit components provided internally.
According to a further preferred embodiment of this invention, the sealing for an attaching portion of the cover member is made by an adhesive seal.
With this embodiment, the unit is further reduced in size by employing the insulating cover member as the outside wall, while the attaching portion of this cover member is sealed more reliably, and the unit is secured for the high water resistance. This is because when the cover member made of synthetic resin is employed as the outside wall, there is the advantage that the insulation distance is unnecessary to provide. However, the synthetic resin cover is weaker in strength than the metallic cover member, and is difficult to achieve the reliable sealing with the ordinary packing needing a fastening force such as the O-ring. However, with an adhesive seal, the fastening force is unnecessary, and the reliable sealing is securely possible.
According to a further preferred embodiment of this invention, a mounting portion of the baseboard for mounting a large current circuit component containing at least a power back-up capacitor is disposed not to overlap the insulating substrate and the metal substrate.
With this embodiment, the unit is prevented from being larger in a thickness direction because the large current circuit component having relatively large parts and the insulating substrate are overlapped. Therefore, the size of the unit in the thickness direction is kept small, although the structure has a plurality of substrates laminated. As a result, the unit can be reduced in size, and further improved to be mounted on the vehicle.
According to a further preferred embodiment of this invention, the power back-up capacitor is disposed on one end side of the baseboard, and the metal substrate is disposed on the other end side of the baseboard.
With this embodiment, a large quantity of heat produced by the metal substrate is kept preventing from being conducted to the power back-up capacitor. The reliability or life of the power back-up capacitor consisting of an electrolytic capacitor that is weak to heat is improved, contributing to higher reliability or longer life of the whole unit.
The control unit for the electric power steering device of this invention, or the control unit for an on-board device of vehicle, that uses an assist motor linked to a steering system of the vehicle to generate a steering auxiliary torque, has a vent channel allowing the ventilation of the air into and out of the unit formed in a member constituting an outside wall of the unit, the vent channel comprising a breather space partitioned from the inside of the unit, an outside vent hole for communicating the breather space to the outside of the unit with a smaller cross sectional area than that of the breather space, and an inside vent hole for communicating the breather space into the unit, wherein the inside vent hole is fitted with a filter porous to air but impervious to water.
Herein, the xe2x80x9con-board device of vehiclexe2x80x9d means any device or system to be mounted on the vehicle, like the-electric power steering device, and may be an engine control system, an AT control system, an anti skid braking system (ABS system), or a power window device, for example.
The xe2x80x9cminute cross sectional areaxe2x80x9d means a cross sectional area of flow path which is so minute that a pressure difference between the inside and the outside of the unit may be sufficiently relieved, owing to the air within the breather space being brought through the inside vent hole into the inside of the unit, as required, while the water entering through the outside vent hole is filled in large quantity in the breather space to enclose the inside vent hole, when the unit is submerged under water. The xe2x80x9cbreather spacexe2x80x9d means a space having a sufficient capacity for such purposes.
With this embodiment, the internal pressure within the unit is prevented from dropping (or at least excessively below) below the external pressure owing to, for example, a reduction in inner pressure by cooling or a water pressure produced by the submerging water, when the unit is submerged under water. Hence, the water resistance can be prevented inexpensively and reliably from degrading owing to the pressure difference between the inside and the outside of the unit.
In the case where the unit has no vent channel with the breather space, but is simply of the air tight structure, it is requisite to have a seal structure for retaining the air tightness to withstand the pressure difference between the inside and the outside of the unit, which is expensive and takes a considerable space (for example, a simple adhesive seal is unusable), inevitably resulting in the larger size and increased costs. If the vent channel is provided with a filter alone but without the breather space, an entrance of this vent hole is covered with water, it being apprehended that the air can be passed into or out of the unit to adjust the pressure difference between the inside and the outside of the unit. As a result, the water resistance can not be necessarily assured owing to the pressure difference between the inside and the outside of the unit.